legacycentral bbs

So I recently decided to switch to a single tip exhaust and ended up finding a catback off of a 98 legacy outback and paired it with some dual port headers and cat section from another legacy outback. When attempting to connect the catback to the cat, I found the new walker donut gasket is too small to fit onto the cat, and the old gasket that was on the cat is too big to fit inside the catback. Is this just because I suck at life? Should I try a genuine Subaru gasket? Any suggestions will help. Thanks in advance

measure the catback flange and see what size it is. I know the aftermarket gaskets vary in size.
check out the size of the bosal one 256-305 sold at autozone etc.. $7
If memory serves me correct that is the one that fit